1. Technical Field
The invention relates to a light source device, an imaging apparatus and an endoscope apparatus.
2. Description of the Related Art
An endoscope apparatus that has been widely used is configured so that illumination light from a lamp that is provided in a light source device is guided by a light guide that is provided along an endoscope insertion portion, and the illumination light guided by this light guide is emitted from an illumination window that is provided at a front end of the endoscope insertion portion so as to illuminate an object site to be inspected. On the other hand, JP 2005-205195 A describes a light source device in which a blue laser beam is guided to a front end side of an endoscope insertion portion by an optical fiber, and a phosphor disposed at a front end of the optical fiber is excited by the blue laser beam to emit light and irradiate white illumination light. Since this light source device has an outer diameter that is narrower than a fiber bundle of the related art, this light source device can be suitable for use in the case where an endoscope is required to have a narrow outer shape as in a transnasal endoscope. However, in this case, the illumination light runs short of an intensity of light around 450 to 480 nm and thus is poor in color rendering property as compared to a continuous spectrum, over a wavelength 430 to 680 nm, of an illumination light (Xe lamp), of the related art, for an endoscope.
Also, in an endoscopic diagnosis, there is a method called a special light diagnosis utilizing an image obtained by illuminating with light in a specific wavelength band, in addition to observation by using the white illumination light (see JP 2001-170009 A and JP 2005-198794 A, for example). In those cases, light in a specific narrow wavelength band is employed as illumination light. The special light diagnosis, for example, can clearly observe a nascent blood vessel produced in a mucosal layer or a mucosal underlying layer, and also can depict the fine structure of a mucosal surface that cannot be obtained in an ordinary observation image. Therefore, this special light diagnosis is beneficial for a diagnosis of lesion, an early detection of cancer, and the like.
Meanwhile, in order to emit light in a specific wavelength band for the special light diagnosis in addition to white illumination light that is obtained by a combination of the laser beam and the phosphor, light in the specific wavelength band and excitation light of the phosphor may be coupled into an optical fiber. FIG. 12A shows an excitation spectrum and an emission spectrum under illumination with normal light (illumination by the white illumination light), while FIG. 12B shows an excitation spectrum and an emission spectrum under illumination with special light (illumination by light in the specific wavelength band). As shown in FIG. 12A, the excitation light has a wavelength in an excitation wavelength band W in which the phosphor is excited to emit light, and this excitation light excites the phosphor so as to emit light having a wavelength component indicated by the emission spectrum. However, as shown in FIG. 12B, under the illumination with in the special light, when the light in the specific wavelength band is light having a wavelength within the excitation wavelength band W, unnecessary fluorescence is emitted from the phosphor, and thus an observation image peculiar to the specific wavelength band might not be obtained.